Electroplating is the process of covering the articles made up of cheap metals by a thin covering of precious metal such as iron with nickel chromium, silver or gold with one or more of them. Electroplating is done due to one or more of following reasons:
- Electroplating protects the metals against corrosion.
- It gives the shiny appearance to articles.
- Electroplating is also used for giving reflecting properties to reflectors.
- Electroplating is used in repair or reconstruction work.
- As an intermediate manufacturing process.
The following conditions must be satisfied for electroplating:
- The article to be electroplated must possess a chemically clean surface, i.e it should not have any rust, grease or dirt.
- The article to be electroplated should form the cathode.
- The metal to be deposited in electroplating process must be present in the electrolyte.
- In order to keep the strength of the electrolyte solution constant during electrolysis, the anode must be of the metal to be deposited.
Process of Electroplating
The object to be electroplated is thoroughly cleaned, polished, degreased and arranged as the cathode in a volta-meter, containing anode and electrolyte of the metal to be deposited. If the electroplating is done on an unclean and greased surface the metal deposited will not be well adherent to the base metal and is likely to peel off.
The cleaning processes involved for an object before electroplating sequence are:
- Sand Blasting: To remove oxide layer by projecting fine or steel ball on the surface of the object by means of a jet of compressed air.
- Grinding: By leather wheel of size 6″, 8″ and 10″dressed with glue and emery on the periphery of the wheel.
- Polishing: By mops made of leather, canvas or felt dressed with glue and emery. The mops are driven at the peripheral speed of 2000 to 3000 meters/minute.
- Buffing: A polishing process usually done by flexible wheels of cotton and a suitable abrasive of the polishing composition. Buffing is done to make the surface of the metal to be electroplated finer.
- Physical Cleaning: To degrease the metal with solvents like white spirit, solvent naphtha etc. at room temperature.
- Chemical Cleaning: By passing the object through alkaline cleaners (in powder form or jelly) dissolved is water and stored in mild steel tanks.
- Acid Dips: To remove films of oxides etc. from the surface of the object. Standard acid dips are 10% HCL and 10% H2SO4.
- Electro-Cleaning: The article to be cleaned from oil and grease is made cathode and the iron tank or vat filled with an electrolyte solution of the electrolyte and heavy current is passed through the solution. Caustic soda and hydrogen are produced at cathode which removes the grease from the surface of the article. The process is called cathodic cleaning and is applicable to zinc and aluminum. For anodic cleaning, the article is made anode.
Factors governing Electroplating
The electroplating can be used only in the case of a limited number of metals and the factors governing deposition processes are:
- Throwing Power: Throwing power is the term used for indicating the ability to produce a coating of uniform thickness over an object of irregular shape in which the current density is not is not same.
- Nature of Electrolyte: Smooth deposits are obtained from solutions having complex ions e.g. cyanides. Silver from nitrate solution forms coarse deposits while from cyanide solution forms smooth deposits. Therefore, the formation of smooth deposit largely depends upon the nature of the electrolyte used
- Heating of the bath (Temperature): In many electro-deposition processes there is a requirement that the bath should be heated to a high temperature. This can be achieved by using steam or electric heating. At moderate temperatures, the deposits are good.
- Nature of the metal on which deposit is to be made: This factor influences the growth of crystals since it is believed that the operation of crystals is in continuation of these in the base metal
- Addition of Agents: Addition of sulphuric acid to the copper sulphate solution reduces the resistance of the electrolyte and save a certain amount of power. The addition of glucose to zinc sulfate is essential for satisfactory deposition of zinc. The quality of deposit is improved by adding organic compounds such as gums, rubber, alkalis, sugar etc.
- Electrolytic concentration: Increase of concentration of the electrolyte tends to give better deposit because by increasing the concentration of the electrolyte, high current density can be achieved.
Polarization and its effects on electro-deposition
The rate at which the metals are deposited depends upon the current density. If the current density is low, the deposit is coarse and crystalline and at higher current densities the deposit will be uniform. But if the current density is more than the required limit it will result in the electrolysis of water and hydrogen will deposit on the cathode. The hydrogen evolved blackens the base metal which diminishes the rate of metal deposition. This phenomenon is called polarization blackening effect of the base metal can be reduced by agitating the electrolyte.
It is a rectangular welded mild steel tank (also known as plating vat) open at the top. Three brass rods run parallel to each other along the top of the vat, two on the sides and one in the center. The rods on the side are connected to the positive end of supply (called anode bars) and the central rod with the negative end of supply (called cathode bar). The articles which are to electroplated are suspended into the plating solution with the help of copper wires, connected to the cathode bars. The anode bars serve for the support of the anodes which are long pieces of metal suspended so that they can reach into the plating solution and serve as the positive electrode.
When DC supply is applied to the two electrodes, current starts flowing through the electrolyte. The metal ions, (which are positively charged) begin to move towards the article (which is negatively charged) and get deposited on it and the non-metallic ions, being negatively charged, move towards the anode.
Low voltage direct current is used for electroplating and the P.D. used varies from 1 to 16 volts depending upon the nature of electrolyte used and the rate at which the plating is accomplished. The current density used varies with different metals. With high current density, the deposit may be crystalline or powdery and will not adhere well to the cathode. But higher current densities can be employed if the electrolyte solution is rapidly circulated by means of a pump or agitated by blowing in the air.
It is mostly used for plating iron articles to prevent them from rusting. lt is also used as a base coating for silver and nickel plating. The surfaced the article to be electroplated should be perfectly clean. The electrolyte is an acid bath solution made of copper sulfate and sulphuric acid. For copper electroplating solution is made of 150 -200 gms of copper sulfate and 20 to 30 gms of sulphuric acid per 1000 cc of solution. The anode is made of pure copper. The time required for plating is 2 to 3 seconds. The voltage required is 4 to 10 volts per cell. A system of series – parallel arrangement of cells is used so that the circuit voltages of 100 – 200 V can be employed. Current density 200 – 400 amps/m2 and temperature of electrolyte 30 – 50°C is maintained during copper electroplating.
After copper electroplating of articles, the deposit obtained is rough and thick and requires polishing. For getting the thin and smooth deposit, a cyanide bath solution made of copper cyanide, sodium cyanide, sodium carbonate and sodium bisulfate is used.
Nickel does not stick well on iron and steel articles. Therefore, the article is first coated with a film of copper and then nickel is deposited on the copper plating. The electrolyte used is a nickel bath solution made of nick sulfate, nickel chloride, and boric acid. Time for plating is 30 minutes, voltage 5 – 6 volts, current density 250 – 500 Amps/m2 and temperature of the electrolyte is 40 – 60°C. The anode is of pure nickel. Nickel life depends upon copper coating under the nickel coating. The filtration and agitation plant is only used in nickel plating.
Steel tanks with lead busing are used. Lead electrodes are used as anodes. The electrolyte must be heated and the fumes should be exhaust by exhaust fans. The article is first nickel plated and then chromium plated and the life of chromium depends upon the nickel coating under the chromium plating. The chromium bath solution is made of chromic acid and sulphuric acid. For chromium plating 180 – 300 gm of chromic acid and 2 – 3 gms H2SO4 per 1000 cc is used. Current density required is 600 to 5000 amps/m2, time 4 – 8 second, voltage 6 – 10 volts and temperature 50 – 70°C. For more information on the power supply for electrolytic plant click the link.
After copper, nickel or chromium electroplating the article is buffed with fine polishing mops made of cotton rotating at high speeds.
The articles are rinsed or water dipped between every process of cleaning such as physical cleaning, chemical cleaning, acid dips in order to prevent the carrying over of one processing solution to the other. To meet this requirement in every plating shop, special rinsing tanks with running water are extensively used. So ample supply of water, as well as drainage facilities, are to be provided in a plating shop.
Reverse Current Process of Electroplating
In this process the plating current is reversed for a second or so at regular intervals to get following advantages:
- Inferior metal is depleted during the reverse current period.
- The flat level surface is produced.
- Metal surface brightened due to the reversal of current periodically and it eliminates the buffing or polishing operations.
- Polarization effect is reduced without affecting the quality of deposit.
Precautions in Electroplating Process
- The object to be electroplated must be cleaned thoroughly.
- It should not be handled with bare hands. Use a piece of paper or clean rubber gloves.
- All connections must be clean, tight and free from corrosion.
- The object to be electroplated must be connected to the negative terminal of the battery.
- The plates (electrodes) should not touch each other.
- Keep the current constant during the electroplating process.
- The plate must be rinsed in dilute H2SO4 after removing from the electrolyte, otherwise, the deposit will turn black.